This is the wrong answer, because:
It's a technical solution to a medical problem. The only long-term solution is a vaccine. In the meantime, we either destroy our society or have thousands die. A tough choice, and at the moment we seem to be managing to do both.
It assumes a sufficient %age of the population have suitable hardware, and are willing to, and capable of, using this "app". Unlikely.
The chances of developing something reliable and effective on this scale at such short notice, even for these rock-star organisations, is minimal. There will be bugs, and security flaws. It will be pwnd by $badguys.
It's a prime example of how so many see technology they don't understand as "magic", and assume that magic can solve any problem, because it's magic, isn't it...